1. Field of the Invention
The present invention relates to silica glass that is substantially free of chlorine (Cl) and has a superior durability against ultraviolet (UV) light and its method of manufacture.
2. Discussion of the Related Art
Conventionally, reduction projection exposure apparatus called a stepper is often used for exposing and transcribing integrated circuit patterns onto a wafer, such as a silicon wafer, using photolithographic technology.
The optical system of this stepper (reduction projection exposure apparatus) is mainly constructed of an illumination optical system and a projection optical system. The illumination optical system is used, to radiate the light source beam uniformly onto a reticle where an integrated circuit pattern is drawn. The projection optical system is used to reduce the image of the integrated circuit pattern, and project and transfer the reduced image onto the wafer.
Due to recent trends towards greater integration of LSI, it has become necessary for the pattern transcribed on the wafer to have a higher resolution. Therefore, as the light source of the stepper, shorter wavelengths, from the g-line (436 nm) and the i-line (365 nm) to excimer lasers, such as KrF (248 nm) and ArF (193 nm), are increasingly being used
Conventional optical glass used as the lens for these illumination optical systems and projection optical systems has a problem of its low light transmittance in the short wavelength range below the i-line. Currently, synthesized silica glass is used as the lens material instead of conventional optical glass.
This silica glass can be manufactured (synthesized) for example, by vapor-phase synthesis called the xe2x80x9cdirect method.xe2x80x9d This direct method include the following steps, for example.
(1) The step for expelling a gaseous silicon compound as a material, an oxygen gas, and a hydrogen gas from a burner made of silica glass. In general, the gaseous silicon compound is diluted with a carrier gas (an oxygen gas, for example) upon emission.
(2) The step for generating minute silica glass particles (soot) by reacting the gaseous silicon compound and water which is a reaction product of the oxygen and hydrogen gases.
(3) The step for depositing the minute silica glass particles on a target.
(4) The step for creating silica glass (lump) by fusing and vitrifying the minute silica glass particles deposited on the target.
It has been discovered that the silica glass manufactured with this manufacturing method using silicon tetrafluoride gas as a gaseous silicon compound is substantially free of chlorine in the silica glass and exhibits a high durability against ultraviolet light, as compared with silica glass manufactured using silicon tetrachloride as the material.
However, when silica glass is synthesized using the silicon tetrafluoride (SiF4) gas as the material, a new problem emerges as a trade-off for the high durability to ultraviolet light. The problem is that the uniformity in refractive index tends to deteriorate in the resultant silica glass.
Accordingly, the present invention is directed to a silica glass and its manufacturing method that substantially obviate the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a silica glass having a superior ultraviolet light durability and a uniform refractive index profile.
Another object of the present invention is to provide a method for manufacturing a silica glass having a superior ultraviolet light durability and a uniform refractive index profile.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the present invention provides a silica glass that is substantially free of chlorine (Cl), including a fluorine (F) concentration of about 100 ppm to about 450 ppm, a difference between the maximum refractive index and the minimum refractive index of the silica glass (xcex94n) being within the range of about 1.0xc3x9710xe2x88x927 to about 1.0xc3x9710xe2x88x925.
In another aspect, the present invention provides a silica glass having a chlorine concentration of about 0.1 ppm or less and a fluorine concentration of about 100 ppm to about 450 ppm, the silica glass having a substantially uniform refractive index distribution with respect to ultraviolet light such that a difference between the maximum refractive index and the minimum refractive index is within the range of about 1.0xc3x9710xe2x88x927 to about 1.0xc3x9710xe2x88x925.
In another aspect, the present invention provides a method for manufacturing a silica glass that is substantially free of chlorine, the method including the steps of separately expelling a silicon tetrafluoride gas, a combustion gas, and a combustible gas from a burner made of silica glass, the flow velocity of the silicon tetrafluoride gas being within the range of about 9 slm/cm2 to about 20 slm/cm2; producing minute silica glass particles by reacting the silicon tetrafluoride gas with water produced by a reaction of the combustion gas with the combustible gas; depositing the minute silica glass particles on a target; and producing the silica glass by fusing and vitrifying the minute silica glass particles deposited on the target
In a further aspect, the present invention provides a method for manufacturing a silica glass, the method including the steps of expelling a combustion gas, a combustible gas, and a material gas including a silicon tetrafluoride gas separately from a burner, the flow velocity of the silicon tetrafluoride gas included in the material gas being within the range of about 9 slm/cm2 to about 20 slm/cm2; reacting the combustible gas with the combustion gas to produce water; reacting the silicon tetrafluoride gas with the water to produce and deposit minute silica particles on-a target; and fusing and vitrifying the minute silica particles deposited on the target to produce the silica glass.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide her explanation of the invention as claimed.